1. Field of the Invention
This invention relates to an electrostatic recording member on which information on a radiation image obtained by exposing an object to radiation such X-rays, is recorded as an electrostatic charge pattern (an electrostatic latent image), an electrostatic latent image recording apparatus for recording image information on the recording member, and an electrostatic latent image read-out apparatus for reading out image information from the recording member.
2. Description of the Related Art
There has been known an X-raying system in which a photoconductive member such as a selenium plate sensitive to X-rays is employed as a photosensitive member (electrostatic recording member) in order to reduce the dose to the object and to improve diagnostic performance, and the electrostatic latent image formed on the selenium plate by X-rays is read out by use of a laser beam or a plurality of electrodes. See, for instance, U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982 and 4,535,468, "23027 Method and device for recording and transducing an electromagnetic energy"; Research Disclosure, June 1983, Japanese Unexamined Patent Publication No. 9(1997)-5906, U.S. Pat. No. 4,961,209, and "X-ray imaging using amorphous selenium"; Med Phys. 22(12).
Such systems are advantageous over a known fluoroscopy using a TV image pickup tube, in that the resolution is higher, and over xeroradiography in that the dosage of X-rays is smaller.
However these systems are disadvantageous in that when the thickness of the selenium plate is increased to compensate for low X-ray absorption of the selenium plate, the image read-out speed is lowered, and they are apt to pick up noise due to geometric accuracy in the direction of thickness of the selenium plate (structure noise).
Further in the systems disclosed in U.S. Pat. Nos. 4,176,275, 5,268,569 and 5,354,982, since the X-ray sensitive photoconductive layer for recording which requires a relatively large thickness of the photoconductive layer is used as the photoconductive layer for read-out, the read-out speed is low, and since a high electric voltage is applied to the photoconductive layer at least until read-out is finished after formation of a latent image, an electric charge due to a dark current is added to the charge of the latent image and the contrast deteriorates in the low dosage area. Further there exists an inherent problem that an erasing process is required for rerecording.
In the systems disclosed in Research Disclosure, June 1983 and Japanese Unexamined Patent Publication No. 9(1997)-5906, the read-out speed is improved by providing a photoconductive layer for read-out separately from that for recording, and the erasing process is unnecessary. However since a high electric voltage is applied to the photoconductive layer until read-out is finished after formation of a latent image (typically for several seconds), an electric charge due to a dark current is added to the charge of the latent image and the contrast deteriorates in the low dosage area.
Further there has been disclosed an approach in which the latent image is read out by short-circuiting the electrodes on the recording side and the read-out side without applying a high voltage upon read-out. However in this approach, a uniform charge distribution is formed in advance by a stored charge on an intermediate interface due to uniform exposure before exposure to X-rays, and a high electric voltage is applied to the photoconductive layer for read-out by the stored charge upon short-circuiting. Accordingly, due to leak of the charge by the dark current, the contrast deteriorates in the low dosage area also in this approach.
Further in the systems disclosed in Research Disclosure, June 1983 and Japanese Unexamined Patent Publication No. 9(1997)-5906, a flood light source for primary exposure is required, which adds to the cost and the size of the apparatus, and it is difficult to realize uniform flood lighting and it is difficult to time the flood lighting to X-raying.
In U.S. Pat. No. 4,535,468, there is disclosed a system comprising a three-layer electrostatic recording member consisting of an X-ray sensitive photoconductive layer, a photoconductive layer for read-out and an intermediate layer on which an electric charge is stored as a trap. In this system, a latent image charge is stored on the intermediate layer by exposing the X-ray sensitive photoconductive layer to X-rays while applying a high electric voltage thereto and then the latent image is read out by short-circuiting the electrodes. In this system, the read-out speed can be increased by use of a thin photoconductive layer for read-out. Further since the high electric voltage is removed after the X-ray latent image is stored on the intermediate layer and short-circuiting is held during read-out, though the latent image charge leaks (attenuates) due to a dark current, the leak is proportional to the amount of the latent image charge unlike the aforesaid system, whereby deterioration in the contrast in the low dosage area can be suppressed.
However since a high electric voltage is generally kept applied for a long time even prior to exposure to recording X-rays, an offset charge component due to a dark current generated by the high electric voltage applied before exposure to the recording X-rays remains and cannot be removed. Further there is a problem that the amount of the signal charge which can be detected from the exterior is small since the photoconductive layer for read-out is smaller in thickness than the X-ray sensitive photoconductive layer. Further since the mobility of charge is low for both electrons and holes in the intermediate layer, the intermediate layer cannot be large in thickness. This is because a high mobility of charge can result in deterioration in response and/or generation of an afterimage. That is, in this system, it is difficult to simultaneously obtain a high response, a large charge storage ability and efficient taking up of the signal charge.
In any of the aforesaid systems, since recording is effected by scanning the electrostatic recording member with a beam spot such as of a laser beam, places where latent image charges are stored, i.e., positions of pixels, cannot be fixed and it is difficult to effect so-called structure noise correction.